In an image display device, for example, a liquid crystal display etc., the image is displayed by arranging a large number of pixels in a matrix and controlling the intensity of light for each pixel in accordance with image information to be displayed.
The same is also true for an organic EL display etc., but an organic EL display is a so-called self light emission type display having a light emitting element in each pixel circuit and has the advantages that the viewability of the image is high in comparison with a liquid crystal display, no backlight is necessary, the response speed is fast, and so on.
Further, this is very different from a liquid crystal display in the point that the luminance of each light emitting element can be controlled by the value of the current flowing through it so as to obtain scales of color, that is, each light emitting element is a current controlled type.
In an organic EL display, in the same way as a liquid crystal display, the simple matrix system and the active matrix system are possible as the method for driving the same. The former is simple in structure, but has the problems that realization of a large sized and high definition display is difficult and so on. Therefore, there has been much development work on the active matrix system for controlling the current flowing in the light emitting element inside each pixel circuit by an active element provided inside the pixel circuit, generally a TFT (thin film transistor).
FIG. 1 is a block diagram showing the configuration of a general organic EL display device.
This display device, as shown in FIG. 1, has a pixel array 2 comprised of pixel circuits (PXLC) 2a arranged in an m×n matrix, a horizontal selector (HSEL) 3, a write scanner (WSCN) 4, data lines DTL1 to DTLn selected by the horizontal selector 3 and supplied with data signals in accordance with the luminance information, and scanning lines WSL1 to WSLm selected and driven by the write scanner 4.
Note that the horizontal selector 3 and the write scanner 4 are sometimes formed on polycrystalline silicon or formed on the periphery of the pixels by MOSIC etc.
FIG. 2 is a circuit diagram showing an example of the configuration of the pixel circuit 2a of FIG. 1 (see for example Patent Documents 1 and 2).
The pixel circuit of FIG. 2 has the simplest circuit configuration among the large number of circuits proposed and is a circuit of the so-called two-transistor drive system.
The pixel circuit 2a of FIG. 2 has a p-channel thin film field effect transistor (hereinafter referred to as an TFT) 11 and TFT 12, a capacitor C11, and a light emitting element constituted by an organic EL element (OLED) 13. Further, in FIG. 42, DTL indicates the data line, and WSL indicates the scanning line.
The organic EL element has a rectification property in many cases, so sometimes is called an OLED (organic light emitting diode). The symbol of a diode is used as the light emitting element in FIG. 2 and other figures, but a rectification property is not always required for the OLED in the following explanation.
In FIG. 2, a source of the TFT 11 is connected to a power supply potential VCC, and a cathode of the light emitting element 13 is connected to a ground potential GND. The operation of the pixel circuit 2a of FIG. 2 is as follows.
Step ST1:
When the scanning line WSL is in a selected state (low level here) and a write potential Vdata is supplied to the data line DTL, the TFT 12 becomes conductive and the capacitor C11 is charged or discharged, and a gate potential of the TFT 11 becomes Vdata.
Step ST2:
When the scanning line WSL is in a non-selected state (high level here), the data line DTL and the TFT 11 are electrically disconnected, but the gate potential of the TFT 11 is held stably by the capacitor C11.
Step ST3:
The current flowing in the TFT 11 and the light emitting element 13 becomes a value in accordance with a voltage Vgs between the gate and source of the TFT 11, and the light emitting element 13 continuously emits light with a luminance in accordance with the current value.
As in above step ST1, the operation of selecting the scanning line WSL and transferring the luminance information given to the data line to the inside of the pixel will be called “writing” below.
As explained above, in the pixel circuit 2a of FIG. 2, when once writing the Vdata, during the period up to when next rewriting the data, the light emitting element 13 continues emitting light with a constant luminance.
As explained above, in the pixel circuit 2a, by changing the gate voltage of the drive transistor constituted by the TFT 11, the value of the current flowing in the EL light emitting element 13 is controlled.
At this time, the source of the p-channel drive transistor 11 is connected to a power supply potential VCC, so this TFT 11 is constantly operating in the saturated region. Accordingly, it becomes a constant current source having a value shown in the following equation 1.
(Equation 1)Ids=½·μ(W/L)Cox(Vgs−|Vth|)2   (1)
Here, μ indicates the mobility of a carrier, Cox indicates a gate capacitance per unit area, W indicates a gate width, L indicates a gate length, Vgs indicates a gate-source voltage of the TFT 11, and Vth indicates a threshold value of the TFT 11.
In a simple matrix type image display device, each light emitting element emits light only at an instant when it is selected, but in contrast, in an active matrix, as explained above, the light emitting element continues emitting light even after the end of writing, therefore this becomes advantageous especially in a large sized and high definition display in the point that a peak luminance and a peak current of the light emitting element can be lowered in comparison with the simple matrix.
FIG. 3 is a diagram showing aging of the 10 current-voltage (I-V) characteristic of an organic EL element. In FIG. 3, the curve indicated by a solid line indicates the characteristic at the time of an initial state, and the curve indicated by a broken line indicates the characteristic after the aging.
In general, the I-V characteristic of the organic EL element deteriorates when time passes as shown in FIG. 3.
However, the two-transistor drive of FIG. 2 is a constant current drive, therefore a constant current continuously flows in the organic EL element as explained above. Even when the I-V characteristic of the organic EL element deteriorates, the light emission luminance thereof will not deteriorate by aging.
The pixel circuit 2a of FIG. 2 is configured by a p-channel TFT, but if it could be configured by an n-channel TFT, it would become possible to use a usual amorphous silicon (a-Si) process in TFT fabrication. By this, a reduction of the cost of the TFT substrate would become possible.
Next, a pixel circuit replacing the transistor by an n-channel TFT will be considered.
FIG. 4 is a circuit diagram showing a pixel circuit replacing the p-channel TFT of the circuit of FIG. 2 by an n-channel TFT.
A pixel circuit 2b of FIG. 4 has an n-channel TFT 21 and TFT 22, a capacitor C21, and a light emitting element constituted by an organic EL element (OLED) 23. Further, in FIG. 4, DTL indicates the data line, and WSL indicates the scanning line.
In this pixel circuit 2b, a drain side of the drive transistor constituted by the TFT 21 is connected to the power supply potential VCC, and the source is connected to an anode of the EL element 23 to thereby form a source-follower circuit.
FIG. 5 is a diagram showing operation points of the drive transistor constituted by the TFT 21 and the EL element 23 in the initial state. In FIG. 5, an abscissa indicates a drain/source voltage Vds of the TFT 21, and an ordinate indicates a drain/source current Ids.
As shown in FIG. 5, the source voltage is determined by the operation points of the drive transistor constituted by the TFT 21 and the EL element 23. The voltage thereof has a different value according to the gate voltage.
This TFT 21 is driven in a saturated region, therefore a current Ids having a current value shown in the above equation 1 flows concerning Vgs with respect to the source voltage of the operation point.
Patent Document 1: U.S. Pat. No. 5,684,365
Patent Document 1: Japanese Patent Publication (A) No. 8-234683